1. Field of the Invention
The present invention relates to a plate type heat pipe and a cooling system using same for cooling parts to be cooled such as semiconductor chip or the like mounted on a printed board.
2. Description of the Related Art
Electronic parts such as semiconductor chips or the like generate heat to some extent, when used, which are mounted on various devices such as personal computers and electric or electronic devices such as power equipment. The function of the electronic parts deteriorates and the life time thereof is shortened, when the electronic parts are over heated, thus the cooling of the electronic parts becomes an important technical issue. There are known cooling methods for cooling heat generating parts which require cooling, for example, which takes in a cool air from outside and removes a hot air within a box receiving heat generating parts by means of a fan, or which cools heat generating parts by attaching a heat transfer block (i.e., cooling body) to transfer heat from the heat generating parts to the heat transfer block.
As the heat transfer block, there may be used metallic material such as copper or aluminum which has high heat transfer property, or ceramics material such as carbon, or aluminum nitride which has excellent heat transfer property. In addition, a heat pipe may be used instead of the heat transfer block.
FIG. 9 shows a cross sectional view of a conventional plate type heat pipe which is applied to cool heat generating parts. As shown in FIG. 9, the plate type heat pipe 7 is installed in such manner that one plate member thereof faces heat generating parts 52 mounted on a printed board 54. The heat generating parts 52 are thermally contacted with the plate member of the plate type heat pipe 7. The heat pipe 7 receives heat from the heat generating parts 52 to cool heat generating parts 52. In the example shown in FIG. 9, a heat dissipating fin 51 is further attached on the other plate member of the plate type heat pipe 7 to further improve heat dissipation from the plate type heat pipe 7 to outside. Reference numeral 53 is a lead (frame) of the heat generating parts (in semiconductor chip). Reference numerals 50 and 501 are a container and a hollow portion of the plate type heat pipe 7, respectively. Working fluid and the like within the hollow portion 501 are not shown in FIG. 9.
The heat pipe is briefly described hereunder. In general, the heat pipe includes a hermetically sealed hollow portion and working fluid received therein. The heat is transferred by the phase transition and movement of the working fluid received within the hollow portion. The inner pressure of the hollow portion is reduced so as to accelerate the phase transition of the working fluid.
When heat is applied to a part (which is often called as a heat absorbing side or portion) of the container forming the heat pipe, the heat is transferred to the working fluid staying in the corresponding portion to the part of the container within the hollow portion to cause the working fluid to evaporate. The evaporated working fluid moves within the hollow portion to a portion (which is often called as a heat dissipating side or portion) in which the evaporated working fluid is cooled to return to a liquid phase, for example the portion to which the heat dissipating fin is attached. The working fluid returned to the liquid phase moves back to the heat absorbing side of the container (i.e., circulation). The heat is thus transferred by the phase transition and movement of the working fluid.
In order to secure the continuous circulation of the working fluid, the heat pipe is placed in such manner that the heat absorbing side is positioned below the heat dissipating side. When placed as described above, the working fluid returned to the liquid phase moves back by the function of gravity. However, the electric or electronic devices such as a note book type personal computer, for example, are sometimes largely inclined or positioned in reverse, when used. The circulation of the working fluid by the function of gravity may not be expected in the above-mentioned situation.
A wick to perform capillary action is installed within the hollow portion of the heat pipe to supplement the function of the heat pipe in the above-mentioned situation. The wick includes a metallic mesh and a metallic wire, and fine grooves are formed in the inner wall of the container to perform the capillary action.
As material for the container of the heat pipe, there are often used material of aluminum, copper and stainless steel and the like. A whole body of the container is not necessarily formed by the same material. In addition to water, substituted Freon or alcohol is used as the working fluid.
In the cooling system of the embodiment as shown in FIG. 9, it is important to contact the plate type heat pipe 7 with the heat generating part 52 (for example, semiconductor chip) which is to be cooled in least heat resistance, in order to accomplish the effective cooling.
When a plurality of heat generating parts to be cooled are mounted on the printed board, the heights of the heat generating parts are different each other in many cases since different kinds of heat generating parts are often mounted. In those situation, it is not easy to cause the plate type heat pipe 7 as shown in FIG. 9 to contact with the heat generating parts. Accordingly, the present inventors invented the plate type heat pipe having protruding portions corresponding to the heights of the heat generating parts to be cooled and filed a patent application directed thereto. As shown in FIG. 10, the heat generating part 62 and the heat generating part 620 have different heights from the printed board 64. According to the plate type heat pipe 8, the heat generating parts 62, 640 having different heights can be effectively cooled. Furthermore, a space for placing wiring is often required between the adjacent heat generating parts 62, 640. According to the plate type heat pipe 8 as shown in FIG. 10, the above-mentioned space can be obtained. Reference numerals 60, 601 show the container, and the hollow portion, respectively. Reference numeral 61 shows the heat dissipating fin, and reference numerals 63, 64 show the lead (frame) and the printed board respectively.
When the cooling systems as shown in FIGS. 9 and 10 are used, it is necessary to have highly preciseness in size to cause the plate type heat pipe to contact with the heat generating parts to be cooled in least heat resistance, even though a heat transfer grease is placed therebetween. More specifically, there are required the preciseness in size of the portion of the plate type heat pipe 7, 8 corresponding to the heights of the heat generating parts 52, 62, as well as flatness of the contact surface thereof. For example, such preciseness as being up to 0.01 mm is often required in relation to the heights of the respective protruding portions of the plate type heat pipe 8.
The thickness of the plate member of the container 50, 60 forming the plate type heat pipe 7, 8 is preferably smaller as far as possible from the point of heat transfer property. Since the size of the printed board 54, 64 is about several cm x several cm in area, the size of most of the plate type heat pipe is often the same as those of the printed board. The thickness of the plate member of the container 50, 60 of those heat pipe 7, 8 is up to 1 mm which is sufficient in strength (when copper plate material is used for the container). More specifically, the thickness of the plate member of the container 50, 60 forming the plate type heat pipe 7, 8 is very small (i.e., thin) in general.
However, when the container 50, 60 having such thin plate member as described above is applied, it is very difficult in cost to realize the highly preciseness in the plate type heat pipe 7, 8 (i.e., expensive). Because the thin material has inclination to be easily deformed, in addition, it is difficult to prevent the container 50, 60 from being deformed by heat according to workings such as welding or the like when the container is fabricated. In addition, the container fabricated by the thin plate material has inclination to be deformed by the raised inner pressure caused together with vaporization of the working fluid.
An object of the present invention is therefore to provide a plate type heat pipe which may contact with the heat generating parts to be cooled in least resistance and enable effective cooling of the heat generating parts, and the cooling system using same.